The present invention relates to a color image forming apparatus such as a color printer, a color copier, or a color facsimile using electrophotography for forming a color image. More particularly, the invention relates to a color image forming apparatus in which a plurality of toner images respectively formed by a plurality of single-color toner image formers are sequentially transferred on a transferring member or a recording medium held by the transferring member.
In general, an image forming apparatus and in particular, a tandem image forming apparatus using electrophotography has a photoconductor as an image supporting member having a photosensitive layer on the outer peripheral surface, a charger for uniformly charging the outer peripheral surface of the photoconductor, an exposing member for selectively exposing the outer peripheral surface charged uniformly by the charger for forming an electrostatic latent image thereon, and a developer for providing toner as a developing powder to the electrostatic latent image formed by the exposing member for providing a visible image (toner image).
Two types of tandem image forming apparatuses for forming a color image are known. In one type, a plurality of (for example, four) image supporting members are brought into contact with an intermediate transferring member such as a transferring belt or the like so that toner images on the image supporting members are sequentially transferred to the intermediate transferring member so as to superpose the toner images of a plurality of colors (for example, yellow, cyan, magenta, and black) thereon to provide a color image. In the other type, a recording medium, such as paper, is held and transported on a belt member, a drum member or the like, and toner images on a plurality of image supporting members are sequentially transferred to the recording medium so as to superpose the multiple-color toner images thereon to provide a color image.
In order to realize a good transfer state of toner images (therefore realizing a fine image) in the color image forming apparatus as described above, it is desirable that the circumferential velocity of the image supporting member and that of the transferring member be completely matched with each other. More practically, however, manufacturing errors and varying tolerances are usually introduced into the image supporting member, the transferring member, or the parts making up their driver units at the manufacturing stage. Thus, it is practically impossible to completely match the circumferential velocity of the image supporting member and that of the transferring member.
If the difference between the circumferential velocity of the image supporting member and that of the transferring member varies, for example, if the circumferential velocity of the image supporting member is higher than that of the transferring member at one point in time and the former becomes lower than the latter at another point in time, the transfer state becomes remarkably unstable and a fine image cannot be provided. Particularly, to superpose multiple-color toner images on each other for providing a color image, a color-to-color shift occurs and the image quality is remarkably degraded.
To prevent color shift from occurring, there is proposed a color image forming apparatus wherein a velocity of the transferring member Vt is faster than the circumferential velocity of each image supporting member Vd (Vt greater than Td) in Japanese Patent Publication No. 11-65222A.
As shown in FIG. 5, this color image forming apparatus comprises a conveyor belt 3 placed on a drive roller 1 and a driven roller 2. The conveyor belt 3 is circulated by the drive roller 1 and the driven roller 2, and holds and transports a recording medium P such as paper. Four single-color toner image formers 5, each having an image supporting member 4, are brought into contact with the conveyor belt 3 and sequentially transfer toner images on the plurality of image supporting members 4 to the recording medium P held on the conveyor belt 3.
Here, the velocity of the conveyor belt 3 is faster than the circumferential velocity of each of the image supporting members 4, that is, Vt greater than Vd.
According to the above configuration, each image supporting member 4 acts on the conveyor belt 3 as a brake. Thus, the tensed state of the conveyor belt 3 becomes stable between the drive roller 1 and the image supporting member 4 adjacent to the drive roller 1, but the image supporting members 4 are set to the same circumferential velocity Vd and the tensed state of the conveyor belt 3 becomes unstable between the image supporting members 4.
Therefore, the state of the transferring point from each image supporting member 4 to the recording medium P held on the conveyor belt 3 also becomes unstable and consequently, a good color image is not always provided.
Further, this publication does not describe any drive structures of the image supporting members and the conveyor belt as the transferring member.
Another way of trying to prevent color shift is disclosed in Japanese Patent Publication No. 4-324881A. This publication discloses an image forming apparatus wherein the velocity of an image supporting member is always higher than that of transferring member.
In FIG. 6, numeral 12 denotes an intermediate transferring drum, and four different-color photoconductive drums 11 (Y, M, C, and K) are brought into contact with the intermediate transferring drum 12. A yellow toner image provided by the photoconductive drum 11Y, a magenta toner image provided by the photoconductive drum 11M, a cyan toner image provided by the photoconductive drum 11C, and a black toner image provided by the photoconductive drum 11K are sequentially transferred to the intermediate transferring drum 12 to form a full-color toner image, which is then transferred to a recording medium P.
The intermediate transferring drum 12 is driven by a driver unit 14 and the photoconductive drums 11 (Y, M, C, and K) are driven by driver units 13 (Y, M, C, and K). Each of the driver units 13 (Y, M, C, and K) is provided with a velocity converter 15 (Y, M, C, and K) composed of a motor and gears.
In the related art apparatus shown in FIG. 6, since the driver units of the image supporting members and the driver unit of the transferring member are driven by separate drive sources to set the velocity V (Y, M, C, K) of the image supporting member (photoconductive drum 11) higher than the velocity V1 of the transferring member (intermediate transferring drum 12), the driving structure becomes very complicated and the apparatus size is also increased.
Japanese Patent No. 2686267 discloses an image forming apparatus having a mechanism for transmitting a driving force from an image supporting member to a transferring roller so that the circumferential velocity of the transferring roller becomes higher than that of the image supporting member, but this document teaches an apparatus for forming a single-color image and does not consider any color shift problem involved in forming a full-color image.
It is therefore an object of the invention to provide a color image forming apparatus which stabilizes the state of transferred toner images from respective image supporting members to a transferring member at a transferring point for providing a fine image with no color shift using a simple driving mechanism.
In order to achieve the above object, according to the present invention, there is provided a color image forming apparatus, comprising:
a drive roller and a driven roller;
a looped belt member, stretched and circulated by at least the drive roller and the driven roller so as to have a tensed side and a slack side; and
a plurality of image supporting members, each supporting a single color toner image thereon, and abutting onto the tensed side of the belt member to define a transferring position at which the toner image is transferred onto either the belt member or a recording medium held by the belt member, while being rotated, wherein:
a circumferential velocity of each image supporting member is determined so as to be lower than a circulation velocity of the belt member; and
a circumferential velocity of an image supporting member which is further from the driving roller is lower than a circumferential velocity of an image supporting member which is closer to the driving roller.
In this configuration, since the image supporting member positioned upstream always acts as a brake with respect to a portion of the belt member situated between the adjacent image supporting members, the tensed state of the belt member becomes stable also between the respective image supporting members. Therefore, the condition of each transferring position becomes stable so that a good color image can be obtained.
Moreover, since the tensed state of the transferring belt becomes stable when the image forming apparatus is activated (when the image supporting members and the belt member are driven), an initial tension given to the transferring belt can be lessened. Therefore, if the image forming apparatus is not activated for a long time period, a considerably large tension does not act on the belt member. Consequently, creep deformation of the belt, which adversely affects image formation, is reduced.
In a preferred embodiment, the apparatus further comprises:
a first gear train, which rotates the image supporting members;
a second gear train, which rotates the drive roller to circulate the belt member, while being connected to the first gear train with no branch; and
a single drive source, which drives the first gear train to thereby drive the second gear train.
In this configuration, backlash in the image supporting member drive gear train that might occur due to the fact that the circumferential velocity of the image supporting member is lower than the circulation velocity Vt of the belt member does not occur, so that good color superposing accuracy can be attained.
Also according to the invention, a surface hardness of the belt member may be greater than a surface hardness of the respective image supporting members. Alternatively, or in addition to this surface hardness relationship, a surface roughness of the belt member may be greater than a surface roughness of the respective image supporting members.
In addition, an abrasive may be applied on a surface of the belt member.
In any of the above configurations, whenever the surface of the image supporting member comes in contact with the belt member, because of the difference between the circulation velocity of the belt member and the circumferential velocity of the image supporting member, the surface of the image supporting member which is slightly cut is always refreshed. Therefore, filming is prevented allowing the image quality to be maintained.
Preferably, the color image forming apparatus further comprises a cleaning member which abuts against a part of the belt member which is wound on the driven roller, to remove toner remained on the belt member.
In this configuration, the stretched condition of the intermediate transfer belt becomes stable even at the initial stage of image formation.
Preferably, the color image forming apparatus further comprises a secondary transfer position, formed on a part of the belt member which is wound on the driven roller, at which the toner images transferred from the image supporting members are secondarily transferred to a recording medium. The recording medium passes through the secondary transfer position upward from a lower part of the apparatus.
In this configuration, it is not necessary to provide the individual transfer roller inside of the transfer belt as shown in FIG. 5, so that it is possible to downsize the stretching structure of the transfer belt, thereby reducing the size of the apparatus.
Here, it is preferable that the color image forming apparatus further comprises a fixing section at which the secondarily transferred toner image is fixed on the recording medium. The fixing section is placed above the plurality of image supporting members.
In this configuration, heat or water vapor generated from the fixing section can be prevented from invading into the image forming section, so that it is possible to prevent occurrence of an image failure caused by temperature fluctuation, a registration shift caused by thermal expansion, an image failure caused by dew condensation, sticking of the contact parts, or the like.
In another embodiment according to the present invention, there is provided a color image forming apparatus, comprising:
a transferring member;
a plurality of image supporting members, each supporting a single color toner image thereon, and abutting onto the transferring member to define a transferring position at which the toner image is transferred onto either the transferring member or a recording medium held by the transferring member, while being rotated;
a first gear train, which rotates the image supporting members;
a second gear train, which rotates the transferring member, while being connected to the first gear train with no branch; and
a single drive source, which drives the first gear train to thereby drive the second gear train,
wherein a circumferential velocity of each image supporting member is determined so as to be lower than a circulation velocity of the belt member.
In this configuration, since the circumferential velocity of the transferring member is lower than the circumferential velocity of each image supporting member, the variation in the difference between the circumferential velocity of the respective image supporting members and the circumferential velocity of the transferring member can be minimized. This makes it possible to stabilize the transferring condition of the toner image of each color from each of the image supporting members to the transferring member to provide a fine image.
Furthermore, since both of the first gear train and the second gear train are driven by the single drive source, the mechanical structure is remarkably simplified which allows for the apparatus size to be reduced.
Moreover, backlash in the first gear train does not occur even though the circumferential velocity of the transferring member is greater than the circumferential velocity of the respective image supporting members. Therefore, the above relationship between the rotation velocities can be reliably provided.
Particularly in the tandem-type color image forming apparatus, not only the driving mechanism for the plural image supporting members and the transferring member is remarkably simplified, but also a fine color image with no color shift can be reliably obtained.
In this embodiment, the surface hardness of the transferring member may also be greater than a surface hardness of the respective image supporting members. Alternatively, or in addition to this surface hardness relationship, a surface roughness of the belt member may be greater than a surface roughness of the respective image supporting members.
Furthermore, an abrasive may be applied on a surface of the transferring member.
In any of the above configurations, whenever the surface of the image supporting member comes in contact with the transferring member, because of the difference between the circumferential velocity of the transferring member and the circumferential velocity of the image supporting member, the surface of the image supporting member which is slightly cut is always refreshed. Therefore, the filming is prevented so that the image quality is maintained.